1. Field of the Invention
This invention relates to an information recording medium, and more particularly to an improvement of a information recording medium for writing and/or reading information by means of laser beam of high energy density.
2. Description of Prior Arts
Information recording media utilizing a beam of high energy density such as laser beam have been developed in recent years and are put to practical use. Such recording medium is generally called "optical disc", and the practical applications thereof have been found, for example, as video disc and audio disc as well as disc memory for large-capacity computer and large-capacity static image file.
Writing of information on the optical disc can be conducted, for example, by irradiating the optical disc with a laser beam. Under irradiation with the laser beam, the irradiated area of the recording layer of the optical disc absorbs the energy of beam, accordingly rise of temperature locally occurs and as a result the recording layer is removed through evaporation or fusion on the area having received the irradiation, whereby the recording of information can be made. Reading of information from the optical disc is also conducted by irradiating the optical disc with laser beam. The information can be reproduced by detecting reflected light or transmitted light corresponding to the alteration in the optical characteristics of the recording layer.
The optical disc generally comprises a disc-shaped transparent substrate of a plastic or glass material and a recording layer composed of a metal or a semi-metal such as Bi, Sn, In or Te which is provided on the substrate.
Between the substrate and the recording layer, a subbing layer or an intermediated layer of a polymer material may be arranged so that the adhesion between the substrate and recording layer, is ensured and the sensitivity of the optical disc is enhanced.
Since the information recording medium is of great value for utilization in various fields of art as mentioned hereinbefore, its sensitivity preferably is as high as possible.
Certain improvements for enhancement of the recording sensitivity have been previously reported: for example, Japanese Patent Publication No. 59(1984)-34519 discloses a laminated composite structure such as a composite of a metal film and PbO, metal fluoride or In-Ge-S chalcogen compound; Japanese Patent Publication No. 58(1983)-33120 discloses a layer comprising a mixture of a metal and GeS; and Japanese Patent Publication No. 58(1983)-15319 discloses a layer comprising a mixture of a metal and a metal fluoride such as MgF.sub.2 or a metal oxide such as Mo0.sub.2.
In these recording media, it is sometimes observed that no pits were formed on the recording layer when a laser beam is applied thereon. The reason appears that while the output energy of the laser beam is sufficient for fusing the recording layer, the fused portion of the recording layer is not well drawn toward the circumferential side for forming a pit and solidifies still in the original fused area. Such recording medium is not satisfactory in the sensitivity, and therefore the output of the laser beam is necessarily increased. Further, in the stage of reading the information recorded in the recording layer, poorly or imcompletely pits sometimes give unfavorable errors in the reproduction of the recorded information.
For improvement of the conventional information recording medium such as enhancement of sensitivity and reduction of the reproduction error, the present inventors have already invented an information recording medium having recording layer which comprises a combination on In and at least one metal compound such as a metal sulfide, a metal fluoride or metal oxide and further contains a metal showing a surface tension of not less than 600 dyne/cm within temperatures of the melting point thereof to a temperature higher than the melting point by 300.degree. C., wherein the concentration of said metal advantageously varying along depth direction of the recording layer in such manner that the concentration of said metal on the side facing the substrate is higher than that of said metal on the opposite side. This invention is disclosed in U.S. patent application Ser. No. 868,126. This recording medium shows enhanced C/N ratio (ratio of carrier/noise) at a low laser output and further shows decreased reproduction error (B.E.R., Bit Error Rate).